This may be due to weak abdominal muscles, liver disease. There is also the so-called "dropsy" - the accumulation of fluid in the abdominal cavity, which is a consequence of liver cirrhosis. The abdomen becomes round, swollen and elastic. Compressing the lungs can provoke circulatory disorders and heart problems. My neighbor died of dropsy, also called Ascites.
Well, it also depends on whether a person drinks or not. Beer bellies are not cleaned up with hard training. Therefore, in the gym there are pumped up people with and without a belly;)
And don't forget about such a thing as heredity. If you are a hypersthenic in the 5th generation, then a mammon is practically provided for you, especially after 35-40.